What Is The Second Step Of Protein Synthesis

mRNA Translation Is The Second Step Of Protein Synthesis

The second step of protein synthesis is mRNA Translation (or just Translation). The mRNA Translation step follows right after the first step of protein synthesis called DNA Transcription (or just Transcription). During the transcription, the information encoded in the DNA is copied to a messenger RNA sequence (mRNA), which then can move through the nucleus membrane and can reach the ribosomes in the cytoplasm. The production of proteins happens during the second step of protein synthesis process – the Translation. Sometimes protein synthesis process is referred only to Translation step, because no actual protein synthesis happens during the Transcription. However transcription is responsible for moving the genetic instructions from the nucleus to the cytoplasm, where the DNA/RNA code is translated by the ribosomes to a polypeptide sequence, which will later be folded into a protein.

The Genetic Code

The genetic code is a set of instructions, which defines how the information encoded within DNA/RNA is translated into proteins. The genetic code is identical (with small exceptions!) among all living organisms. The genetic code determines how codons (sequences of nucleotide triplets) can define which particular amino acid must be added on a specific position within the polypeptide chain. Both DNA and RNA molecules use 4 different building blocks, the nucleotides, to encrypt the genetic information. In order to be able to code for one of the 20 different amino acid residues building polypeptides, the information within the nucleotide sequences must be encoded by a sequence of at least 3 nucleotides, called a triple or codon. Thus using these triplet sequences, the cell is able to define 64 different codes for 20 different amino acids, meaning that for each amino acid there will be more than one responsible codon. This is the so called “codon degeneracy”, which stands for the fact that the genetic code is redundant (one amino acid is coded by more than one codon), but not ambiguous (one codon codes just one amino acid). The code, translating the different codons to amino acid residues, can be described in a simple table with 64 entries, like the one below:

standard genetic code table

Most of the genes are encoded with exactly one and the same code, that is often referred to as standard genetic code. In fact, there are some variant codes, like the synthesis of proteins encoded by the mitochondrial chromosome, which rely on a genetic code that is different from the standard genetic code.

Cellular Machinery Involved In The Second Step Of Protein Synthesis

Different Types of RNA Molecules

The translation process requires the involvement of three different types of RNA molecules, each having it’s own function:trna diagram

  • messenger RNA (mRNA) which acts as a “middle-man” or “messenger” between the DNA which caries the genetic material and the protein production workshops (the ribosomes) which reside in the cytoplasm.
  • different ribosome RNA (rRNA) molecules which participate in the formation of the ribosome sub-units
  • multiple transport RNA (tRNA) molecules for each individual amino-acid residue

Ribosome – The Protein Synthesis Workshop

What Is the Ribosomes Function

Ribosome is the cellular machinery which actually synthesize all cellular proteins. The information encoded in the messenger RNA (mRNA) is transformed by the ribosome into amino acid sequence of the proteins. This cellular organelle consists of specialized RNA molecules called ribosomal RNAs (rRNAs) and multiple different proteins. It is important to be stated that these rRNAs have more structural and regulatory role, rather than to encode the required information about specific proteins, like mRNA does.

Ribosome Structure

The ribosome has two sub-units, which are build by rRNAs and specific ribosomal proteins. These two sub-units have different size, which denotes the one to be called large sub-unit and the other – small sub-unit, respectively. As forming the ribosome, the two sub-units fit to each other and form structure with close to a spherical form, but slightly longer along its axis than in diameter. During protein synthesis, the two sub-units function together to translate the information encoded in mRNA into a polypeptide chain.

The ribosomal subunits slightly differ in prokaryote and eukaryote cells.

Prokaryotic Ribosome

Prokaryotes have 70S ribosomes. Their ribosomes consist of one large (50S) and one small (30S) subunit. Each of the small sub-units is build of a 16S RNA sub-unit which is 1540 nucleotides long. This rRNA is bound to 21 different ribosomal proteins. Similarly, the large sub-unit contains of one 5S RNA of 120 nucleotides and one 23S RNA which is 2900 nucleotides long. The two ribosomal RNA molecules in the large sub-unit are bound to 31 proteins.

Eukaryotic Ribosome

The eukaryotic ribosomes slightly differ from these of the prokaryotes.  They have one large (60S)  and one small (40S) sub-unit, which as a result forming a  80S ribosome. The eukaryotic small sub-unit contains 33 proteins bound to an 18S RNA with a sequence of 1900 nucleotides. The eukaryotes have 3 RNAs in their large subunit –  a 5S RNA (120 nucleotides) , a 28S RNA (4700 nucleotides), and a 5.8S RNA (160 nucleotides) . The 3 ribosomal RNAs are bound to 46 proteins and form the large sub-unit.

Enzymes Involved In Translation

An enzyme called aminoacyl tRNA synthetases is responsible for proper binding between amino acids and the corresponding anticodon of the tRNAs. To be more precise, the aminoacyl tRNA synthetases catalyze the esterification of a specific amino acid (or even its precursor) with one of all compatible tRNA molecules and thus forming an aminoacyl-tRNA. In prokaryiotic cells, EF-Tu elongation factor transfer the aminoacyl-tRNA to the ribosome. In the ribosome, based on the complementary base pairing rule, tRNA anticodons are matched to the corresponding mRNA codons.

The Process Of Translation Is the Second Step Of Protein Synthesis

As already mentioned, the process of translation happens in the cytoplasm, where mRNA binds with ribosomes, which are the exact protein synthesis sites. Ribosomes have three spacial regions, called binding sites, which play important role in the protein synthesis process. One of these binding sites is responsible for binding of mRNA. The other two domains are used to attach tRNA molecules and are labeled as “A site” and “P site”.

second step of protein synthesis - ribosome diagram

The attachment of mRNA molecule to the ribosome makes possible the binding of tRNA molecules to the ribosome in an order defined by the nucleotide sequence of the mRNA. Each tRNA is associated with specific amino acid. This tRNA function is tRNA is determined by the structure of the molecule itself – tRNAs are cloverleaf shaped polynucleotide sequences. The tRNA tail end has an acceptor stem that can bind a specific amino acid. While its head has three nucleotides that form the so called “anticodon” which recognize the corresponding codon sequence of the mRNA molecule. Or in other words, the tRNA anticodon binds complementary to the nitrogenous bases of the mRNA. To ensure consistency of the protein synthesis process, all tRNA molecules having the same anticodon sequence always carry one and the same amino acid residue.

The process of translation begins when the mRNA molecule binds to the ribosome. The start codon (the first one!) always calls for methionine amino acid. The start codon enters the P site, while the second codon enters the A site. The anticodon of tRNA carrying methionine forms temporary base pair with start the codon. A second tRNA molecule with an anticodon complementary to the mRNA codon in the A site approaches the A site. This tRNA anticodon forms a temporary base pair with the codon of the mRNA in the A site. The amino acid attached to the tRNA in the A site and the methionine in the P site form a peptide bond and thus initiate the polypeptide chain. The ribosome moves down the mRNA sequence and the tRNA that resided in the A site moves over to the P site. Then a new mRNA codon enters in the A site. A tRNA carrying a complementary amino acid connects with the bases of the new codon in the A site. Once done, the two adjacent amino acids form a new peptide in the chain. Again, the ribosome moves down the mRNA sequence. The tRNA molecule from the P site is released into the cytoplasm, where it can bind with another amino acid of the same type. A tRNA complementary to the new codon in the A site enters and a new peptide bond is created between the new amino acid and the currently formed peptide chain. The process of translation repeats until one of the 3 stop codons enters the A site of the ribosome. Once the final peptide bond is created, the protein chain which is connected only to the tRNA located in the P site moves to the cytoplasm.

The process of translation is completed!

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